The prototype of a dye-sensitized solar cell was a photoelectric conversion element or solar cell that Graezel, et al. of Ecole Polytechnique Federale de Lausanne first reported in 1991 (M. Graezel, Nature, 353, 737 (1991)), and generally called a dye-sensitized solar cell, a wet-type solar cell, or a Graezel-type solar cell. The feature of the photoelectric conversion element is that it has high photoelectric conversion efficiency as high as that of amorphous silicon-type solar cells and that its production cost can be lowered.
FIG. 1 is a schematic cross-sectional view showing a typical dye-sensitized solar cell. The dye-sensitized solar cell comprises three parts, i.e., a dye electrode 11, an electrolytic layer 5, and a counter electrode 12. Here, the dye electrode 11 has a structure having a sheet-shaped electrode 9 composed of a substrate 1 and an electroconducting layer 2 having formed thereon a porous metal oxide film 3 (collectively called “photoactive electrode 10”) carrying a sensitizing dye 4 thereon. On the other hand, the counter electrode 12 has a structure having a sheet-shaped electrode 12 composed of a substrate 8 having an electroconducting film 7, the electrode 12 having formed thereon a catalyst layer 6. Here, the catalyst plays a role of promoting reduction reaction of electrolyte in the electrolytic solution when the cell is in service. Generally, platinum, carbon black or graphite is used as the catalyst. The electrolytic layer 5, which is generally composed of a solution of an electrolyte, fills in between the dye electrode and the counter electrode to be in electrochemical contact with both of the electrodes . In this solar cell, irradiation of light on the side of the dye electrode 11 allows feeding electrons through the electroconducting film on the side of the dye electrode to outer circuitry.
Next, the mechanism in which photoelectric conversion takes place in a dye-sensitized solar cell will be described. First, absorption of the light energy injected from outside by the sensitizing dye 4 carried on the dye electrode 11 results in generation of electrons and holes on the sensitizing dye. The generated electrons pass through the metal oxide film 3 and reach the electroconducting film 2, through which they are fed to the outside system. On the other hand, the generated holes are transported through the electrolytic solution 5 to the counter electrode 12, where they are recombined with the electrons fed through the electroconducting film 7.
As can be presumed from the mechanism of photoelectric conversion, the photoelectric conversion efficiency of a dye-sensitized solar cell largely depends on the performance of photoactive electrode. To increase the photoelectric conversion efficiency of a dye-sensitized solar cell, the structure and composition of the metal oxide film are particularly important. Specifically, in order for a dye-sensitized solar cell to act stably, it is necessary to keep fine particles of metal oxide which constitutes the photoactive electrode in a state of contacting with each other. This is because contact of metal oxide fine particles with each other allows photo-induced charge-separated electrons on the sensitizing dye to flow in the metal oxide film efficiently. The kind of metal oxide fine particles and degree of adhesion between the fine particles is also assumed to influence the electron conductivity thereof. In addition thereto, it is effective to make the metal oxide film porous. Using the porous film leads to an increase in area per unit volume of the film, so that the carrying amount of sensitizing dye can be increased. This allows effective use of the light injected into the photoelectric conversion element from the outside to photoelectric conversion. Furthermore, another reason of making the metal oxide film porous is that such allows the solution of the electrolytic layer to diffuse all over the metal oxide film, resulting in that the holes generated on the sensitizing dye can be efficiently transported.
In a conventional production method, to produce a photoactive electrode, a method is used in which a metal oxide dispersion composed of a mixture of metal oxide fine particles and an organic substance which is burned out upon heating, specifically a polymer compound having polyethylene glycol or polypropylene glycol as a main chain is coated on a sheet-shaped electrode by a screen printing method, a doctor blade method, a spin coating method or the like, dried and then heat-treated at a temperature at which the metal oxide is sintered (specifically, at a temperature of 400° C. or higher when titanium oxide is taken as an example). According to this method, many voids formed by the burning out of the organic substance remain in the metal oxide film, which can make the metal oxide film porous and which can bind the metal oxide fine particles to each other. That is, in the conventional technique for fabricating a photoactive electrode, the step of heat-treating the metal oxide film coated on the sheet-shaped electrode at a temperature at which the organic substance is burned out is indispensable. From these reasons, production of solar cells by the conventional technique requires a large amount of heat energy and the sheet-shaped electrode used for the photoactive electrode requires heat resistance, so that the practically usable substrate is limited to fluorine-doped tin oxide glass, which has the feature of high heat resistance but is expensive, heavy and poor in shape freedom. In other words, in accordance with the conventional method, it is difficult to fabricate dye-sensitized solar cells with a substrate of sheet-shaped electrode which is light in weight, inexpensive and flexible, for example, a resin, etc., while maintaining an acceptable photoelectric conversion efficiency or with a sheet-shaped electrode having indium tin oxide as inexpensive electroconducting film.
For the purpose of imparting freedom in shape to dye-sensitized solar cells, there have been several reports on the technique of fabricating dye-sensitized solar cells using sheet-shaped electrode made of a material other than fluorine-doped tin oxide glass, in particular a material having freedom in shape, as a substrate therefore.
For example, there have been reported a method involving sintering a metal oxide at a high temperature by using a metal foil having heat resistance as a sheet-shaped electrode (for example, JP 11-288745 A) and a method involving an anodization or chemical oxidation method (for example, JP 10-112337 A). However, when dye-sensitized solar cells with sheet-shaped electrodes composed of these metal substrates are used, light must be introduced from the side of counter electrode because the metal substrates are opaque. As a result, they have disadvantages of a great decrease in photoelectric conversion efficiency in that the electrolytic layer absorbs most part of light to cause a great energy loss and that the most part of photoelectrons is generated at sites remotest from the sheet-shaped electrode of metal oxide film, so that photoelectrons are restricted in their movement due to electric resistance of the metal oxide. Furthermore, the substrate of the sheet-shaped electrode which can endure corrosion by iodine used as an electrolyte of the dye-sensitized solar cell is limited to expensive metals such as titanium, tantalum, and niobium, so that the dye-sensitized solar cells produced by these methods become expensive.
There has been also a report on the technique of performing sintering of a metal oxide at a low temperature using a flexible resin as a substrate of the sheet-shaped electrode (for example, B. A. Gregg et al., Langmuir, 2000, Vol. 16, 5626). In this case, a resin which is less expensive, more transparent and further endurable to corrosion by the electrolyte than the above-mentioned metals can be used as a substrate of the sheet-shaped dye electrode and hence a dye-sensitized solar cell which takes advantage of the feature of a resin substrate and has high performance is expectable. By this method, formation of dense metal oxide film at low temperatures allows adhesion of metal oxide fine particles to each other without practicing heat treatment at high temperatures. However, since this method fails to make the metal oxide film porous, the fabricated photoactive electrode has a problem in that it has greatly decreased performance as compared with the conventional porous photoactive electrode which is fabricated by the conventional high temperature heat treatment.
Even if a porous metal oxide film were formed by this method, adhesion between the metal oxide fine particles would be insufficient because of absence of sintering treatment at high temperatures and thus the mechanical strength of the metal oxide film would be insufficient. Therefore, it is expected that slight mechanical vibration which the metal oxide electrode receives, temperature variation which the cell is susceptible to or the like deteriorates contact between the metal oxide fine particles. That is, the dye electrode fabricated by this method will not be able to maintain the performance for a long time.
In the case where sintering is performed at low temperatures, the kind of metal oxide also gives a great influence on the performance. Taking titanium oxide as an example, one produced by a method which is called as a wet method generally used for dye-sensitized solar cells, for example, titanium oxide produced through hydrolysis of a metal alkoxide and supplied in the form of a solvent dispersion, is not suitable for the case where sintering is performed at low temperatures. This is because organic substances used in the production process are adsorbed and remain on the surface of such metal oxide fine particles, which deteriorates contact between the fine particles, so that movement of electrons are not performed smoothly, resulting in decreased performance. On the other hand, when the photoactive electrode is sintered at high temperatures, this problem does not occur. This is presumably because the adsorbed organic substances are removed from the surface of the metal oxide fine particles by heating at 200° C. or more, resulting in contact between the fine particles (for example, K. Murakoshi et al., J. Electroanal. Chem., 1995, Vol. 396, 26).